1. Field of the Invention
The present invention relates generally to fiber optic rotation sensing devices such as gyroscopes and more particularly to an improved sensing coil useful in such devices.
2. Description of Related Art
A fiber optic gyroscope comprises the following main components: (1) a light source, (2) a beamsplitter (either a fiber optic directional coupler or an integrated-optics Y-junction), (3) a fiber optic coil, (4) a polarizer (and sometimes one or more depolarizes), and (5) a detector. Light from the light source is split by the beamsplitter into copropagating waves that travel through the sensing coil. Associated electronics measures the phase relationships between the two interfering, counterpropagating beams of light that emerge from the opposite ends of the coil. The difference between the phase shifts experienced by the two beams provides a measure of the rate of rotation of the platform to which the instrument is fixed.
Environmental factors can affect the measured phase shift difference between the counterpropagating beams, thereby introducing a bias error. Such environmental factors include variables such as temperature, vibration (acoustical and mechanical) and magnetic fields. These are both time-varying and unevenly distributed throughout the coil and induce variations in index of refraction and length that each counterpropagating wave encounters as it travels through the coil. The phase shifts imposed upon the two waves due to environmental factors can be unequal, producing a net undesirable phase shift which is indistinguishable from the rotation-induced signal.
A very important fiber optic gyro (FOG) bias mechanism or "environmental factor," first described by Shupe, is the time dependent thermal asymmetry of the optical path. Shupe noted that FOG coil winding patterns that kept the fiber segments that are clockwise (CW) of the fiber midpoint adjacent (co-located) with the matching fiber segments that are counter clockwise (CCW) of fiber midpoint would cancel out the Shupe bias, since then the thermal perturbations would occur equally to the CW and CCW fiber halves and the optical pathlength would remain symmetrical about its midpoint.
Frigo suggested radial dipole and radial quadrupole winding patterns as a way of co-locating matched CW and CCW fiber segments. These winding patterns used a single fiber with the wind starting with the fiber midpoint at the coil inner radius (hub). Then the first layer from one of the two payout spools would be wound by rotating the FOG coil spool CW. The other payout spool would then be used to wind a layer by rotating the FOG coil spool CCW and so on, so that CW and CCW matched segments were always disposed within about one layer of each other.
U.S. Pat. No. 4,793,708 of Bednarz, entitled "Fiber Optic Sensing Coil" teaches a symmetric fiber optic sensing coil formed by duopole or quadrupole winding. The coils described in that patent employ orthogonally wound monofilament fibers and exhibit enhanced performance over the conventional helixtype winding.
Experiments have shown that the quadrupole winding procedure is capable of reducing Shupe bias by a factor of a few hundred with respect to a simple thread wind where one of the fiber ends is at the coil hub. However, even this large reduction factor is not adequate for aircraft accuracy navigation. One of the problems is that the quadrupole wind has a built-in bias in that the first and fourth layers of each quadrupole period are dominant over the second and third layers. A second problem is that the quadrupole coil winding pattern must be without defects in order to achieve its expected performance.